Injectable polyethylene oxide gel implant and method for production

ABSTRACT

A biocompatible poly(ethylene oxide) gel implant and method for production which can be injected into the human body for tissue replacement and augmentation. The implant is prepared by dissolving a sample of essentially pure poly(ethylene oxide) in a saline solution in a sealed canister, removing all free oxygen from the container and replacing it with an inert gas, such as argon, and irradiating the canister with a gamma ray source to simultaneously crosslink the polyethylene oxide while sterilizing it. The gel can then be placed into a syringe and injected into the body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a division of U.S. Ser. No. 08/299,583, filed Sep.1, 1994, which is a continuation-in-part of U.S. Ser. No. 07/836,911,filed Feb. 19, 1992, now U.S. Pat. No. 5,372,580, which is acontinuation-in-part of U.S. Ser. No. 07/551,807 filed Jul. 12, 1990,now U.S. Pat. No. 5,090,955.

BACKGROUND OF THE INVENTION

This invention relates generally to a method for producing apoly(ethylene oxide) implant and, in particular, to a method forproducing a biocompatible crosslinked poly(ethylene oxide) gel which canbe injected into the human body for tissue replacement and augmentation.

It is well known that hydrogels have been used in many biomedicalapplications, as they can be made non-toxic and compatible with tissue.U.S. Pat. Nos. 4,983,181 and 4,994,081, which issued in 1991 toCiverchia, teach a method of polymerizing a hydrogel in the presence ofa crosslinking agent to form a three dimensional polymeric meshworkhaving controlled spacings between the molecules thereof to anchor themacromolecules which have a known size and to insure that themicromolecules will be substantially uniformly interspersed within thepolymeric meshwork of the polymerized hydrophilic monomer. The step offorming the crosslinking of the hydrogel can be performed with acrosslinking agent which may be external, such as ultraviolet radiation,or a crosslinking agent added to the hydrogel clear viscous monomersolution, which crosslinking agent may be, for example, ethyleneglycoldimethacrylate. The hydrogel taught in these patents is a transparentcollagen hydrogel which is capable of promoting epithelial cell growth.

Some of the drawbacks of using collagen gels are that they typicallybiodegrade in three to six months, and are well known for theirinfectious and immunologic reactions. In addition, collagen implantsare, in time, colonized by the recipient cells and vessels.

Another type of substance commonly used in biomedical applications is asilicone gel. However, silicone gels are also known to cause immunologicreactions, and tend to migrate away from the implantation site. Inaddition, silicone implants become encapsulated by dense fibrous tissuescreated by cellular reactions to a foreign substance implanted into thetissue. Finally, while silicone gels do allow for efficient oxygendiffusion, there is insufficient transportation of nutrients across thespace that the implants occupy.

SUMMARY OF THE PRESENT INVENTION

It is therefore an object of the present invention to provide a processfor producing a gel implant which is biocompatible with and nonerodiblein the body.

Another object of the present invention is to provide an implant whichcan be easily removed from the body if desired.

It is also an object of the present invention to provide a biocompatiblegel which is injectable into the body and does not cause infectious,inflammatory, or immunologic reactions following implantation.

It is a further object of the present invention to provide an injectablebiocompatible gel which does not migrate away from the site of theinjection, and allows for both oxygen and nutrient support.

It is a still further object of the present invention to provide apoly(ethylene oxide) gel which can be cracked after gelation but beforeentering the body or during the actual injection process.

These and other objects are accomplished in the present instance byusing a novel process for creating a polyethylene oxide (PEO) gel whichcan be injected into the body as an implant. Using gamma radiationcrosslinking, a PEO gel in deoxygenated saline solution is synthesizedfor use as permanent soft implants for tissue replacement andaugmentation, which is useful in plastic and reconstructive surgery,ophthalmic procedures such as refractive corneal surgery, retinaldetachment surgery, and oculoplastics.

Using this novel process, the PEO gel is biocompatible and itscharacteristics can be engineered by modulating PEO-water concentrationand radiation dosage (to control its transparency and hardness) and bymodulating electrolyte concentration (to control volume expansion andfinal water content) to fit a specific medical requirement. The gel isinjectable through small gauge (e.g. 25 ga) needles, and is foundbiocompatible intrastromally and subcutaneously. The gel is notcolonized by cells and vessels, and is therefore easily removable byflushing using saline solutions (preferably hypertonic). The shape ofimplants composed of this PEO gel is moldable by digital massage of thetissue surrounding the implant.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates pictorially a PEO molecule network;

FIG. 2 is a graphic representation showing the influence of molecularweight on gelification dose.

FIG. 3 is a graphic representation showing the percentage of lighttransmission through both a human cornea and a PEO gel implant preparedby a the present process to the wavelength of light.

FIGS. 4A and 4B illustrate pictorially the reflection of light from animplant within a cornea.

FIG. 5 is a graphic representation showing the percentage of lightreflection from a cornea with an implant in relation to the refractiveindex of the implant; and

FIG. 6 is a diagrammatic view of the cornea, illustrating both thetransverse and radial directions in which the modulus of elasticity ismeasured.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Poly(ethylene oxide) (PEO) and poly(ethylene glycol) (PEG) arefabricated by two different methods, but generally refer to the samepolymeric synthetic product having the formula:

    --(--CH.sub.2 --CH.sub.2 --O--).sub.n --

The difference between these two polymers resides in their respectivemolecular weight. PEGs have molecular weight below a few thousanddaltons, whereas PEOs have molecular weights starting from severalthousands to several million daltons.

PEO is soluble in benzene, freon, chloroform, and tetrohydrofurane, andis also soluble in water at all temperatures except near the boilingpoint. PEO is also soluble in salt solutions.

As the PEO polymer is highly soluble in water, to use it as abiocompatible material, it is necessary to decrease its solubility. Thiscan be done by creating an insoluble crosslinked network, as can be seenin FIG. 1. Each crosslink is indicated by a junction, as shown at 1 inFIG. 1. This network has the advantage to be hydrophilic, and,consequently, it will swell in water.

One method for producing crosslinked PEO is by endlinking the networkwith a chemical reaction by using, for example, hexamethylenediisocyanate as the crosslinking agent and a branching agent such asmannitol, pentaerythrytol or 1,2,6-hexametriol. However, because toxicchemical reagents (in the same concentration range as PEO) are usedduring the crosslinking, an additional purification step must beemployed to eliminate any remaining trace of the reagents.

Another way to create this network is to expose the PEO to gammaradiation. However, while pure PEO can be gamma ray crosslinked withoutwater, the process requires a very high radiation dosage (greater than100 Mrad), making it impractical. By using a PEO-water solution, thecrosslinking can be accomplished using a much smaller radiation dosage(about 1 Mrad). This crosslinking is indirect and involves watermolecules: ##STR1##

The radicals produced react on the PEO polymer chain to yield: ##STR2##

The crosslinked PEO chain has a much higher molecular weight than thebase PEO used in the reaction. If a single link occurs between two200,000 dalton chains a 400,000 dalton molecule is obtained. A link canoccur between any two carbon moieties of any two different PEO moleculesas shown in the above formula. Gelation occurs when there is at leastone crosslink per polymer chain initially present.

Gelation depends on several parameters: the PEO concentration, themolecular weight, and the radiation dose. The influence can berepresented in the chart shown in FIG. 2 showing the radiation dose vs.the PEO concentration in aqueous solution for different molecularweights, where MW1>MW2>MW3>MW4. As can be seen in FIG. 2, at a givenconcentration, the higher the molecular weight, the lower the radiationdose necessary to form a gel. However, gelation may not occur, as oxygendissolved in the solution acts as a scavenger of gamma rays and thuswill quench the crosslinking process.

To prevent this, the PEO solution should be carefully degassed. Thesolution is pulled under vacuum until no more bubbles of gas appear inthe solution, then the vacuum is replaced by argon or another inert gas.This procedure may be repeated several times in order to decrease theresidual amount of oxygen remaining in the solution.

In the preferred embodiment, a 0.8% to 8% PEO solution by weight wasprepared by dissolving a PEO preparation (e.g. 200,000 daltons) in asaline solution. The solution used, a Balanced Salt Solution (BSS), wasselected as it is best suited for the intended medical application.Other solutions may be used, depending on the intended use of the gel.The BSS composition, which may be obtained from Alcon, Inc., is listedbelow in Table I.

                  TABLE I                                                         ______________________________________                                        Solute          Percentage (by weight)                                        ______________________________________                                        Sodium Chloride 0.64                                                          Potassium Chloride                                                                            0.075                                                         Calcium Chloride                                                                              0.048                                                         Magnesium Chloride                                                                            0.03                                                          Sodium Acetate  0.039                                                         Sodium Citrate Dihydrate                                                                      0.17                                                          ______________________________________                                    

Free oxygen was then removed from the solution by placing the solutionin a sealed container or canister which was evacuated using vacuum andthen filled with pure Argon gas (>99.999%) to prevent gaseouscontamination from the surrounding atmosphere. Alternatively, the freeoxygen may be removed from the same container in which the PEO solutionhas been prepared. The canister was then irradiated by exposing it to agamma ray source (Cobalt 60) for a dosage of between 2.5 and 25 Mrads tocrosslink the PEO. To obtain a uniform gel (Isotrope) the solution canbe continuously agitated, even during radiation (using a rockingplatform oscillatory shaker). Aseptic and contamination-free transfer ofthe PEO gel to sterile syringes was performed in a laminar flow-hoodpresterilized with UV radiation for use in experimental procedures whichwill be discussed.

It was observed that the PEO hydrogel of a specific electrolyteconcentration, will swell when immersed in a saline solution with alower electrolyte content, while it will shrink if immersed in a salinesolution with a higher electrolyte concentration. Therefore, implantinga PEO gel crosslinked in a saline solution having a differentelectrolyte concentration than surrounding tissue will result in apostoperative change of the implant's volume. While this phenomenon mayresult in postoperative complications in certain medical applications,it can be advantageous in applications such as vitreous substitutionwith polymers and retinal detachment surgery where controlledtissue-to-tissue compression is required.

For a given PEO solute concentration, the higher the irradiation dosage,the higher the crosslink density. Using a 0.8% PEO solution, theirradiation dosage was varied from 0.8 Mrads to over 13 Mrads. 0.8 Mradsseemed to be the minimum dosage required to obtain gelation withoutgravitational collapse of the polymer, while any dosage above 9 Mradsseemed to have little effect on the physical properties of the PEO.

A minimal dose of 2.5 Mrad was selected for the irradiation dosage, asit corresponds to the minimum dosage required for gamma raysterilization. By using a higher dosage, it is possible tosimultaneously crosslink and sterilize the PEO gel implant.

Referring again to FIG. 2, it can be seen that for a given crosslinkdensity, the higher the PEO solute concentration, the lower theirradiation dose required. Initial testing performed with a PEO ofapproximately 200,000 daltons indicated that, below 0.5%, gelation isdifficult to obtain, even at a high irradiation dosage. Thus, a soluteconcentration varying between 0.8% and 8.0% was selected.

With a 0.8% 200,000 dalton PEO solution irradiated at 5 Mrads, thecrosslinked gel is transparent and can be used in ophthalmology forcorneal tissue augmentation procedures such as Gel Injection AdjustableKeratoplasty (GIAK), which is described in U.S. Pat. No. 5,090,955,which is assigned to the same assignee of the present invention and ishereby incorporated by reference.

Visibility of the gel within the eye is a cosmetic and therapeuticconcern related to the GIAK procedure. Gel visibility is relateddirectly to both the reflectivity and absorbance properties of the gelused. Thus, at any visible wavelength, the percentage of transmission oflight through the implant should be at least as great as that throughthe cornea. FIG. 3 shows a graph which illustrates light transmissionthrough both a cornea and an implant prepared according to the presentinvention as a percentage of transmission of light through the cornea asa function of the wavelength of the light. The graph of lighttransmission through the gel is a dotted line designated as 2, while thegraph of light transmission through the cornea is a solid linedesignated as 4. As can be seen in FIG. 3, for the visible lightspectrum (from 400 nanometers to 800 nanometers) the percentage of lighttransmission through the gel approaches 100 percent. Therefore, theimplant of the present invention is optically transparent to lightpassing through the implant. FIG. 3 also shows that the implanttransmits more light in the near ultraviolet, visible and near infraredrange than the normal cornea (wavelengths of 300 to 1350 nm).

As the eye can detect approximately 10% difference in reflection, it isimportant that the index of refraction of the gel differs no more than+10% from the index of refraction of the cornea. FIG. 4A shows a beam oflight passing through an implant which has been placed within the corneaof an eye. A beam 10 passes through the anterior section of cornea 12and strikes the anterior surface 14a of implant 14, where it ispartially reflected as shown at 16. As beam 10 continues through implant14, it strikes the posterior surface 14b of implant 14, and is partiallyreflected as shown at 18.

Referring now to FIG. 4B, the reflection properties of the cornea aretaken into consideration unless a beam passes through a corneacontaining an implant. As beam 10' strikes the anterior surface 20a ofthe tear film 20 of cornea 12', it is partially reflected, as shown at22. Beam 10' continues through tear film 20 and is partially reflectedat anterior surface 12a' of cornea 12', as shown at 24. Beam 10'continues into cornea 12' where it is partially reflected at anteriorsurface 14a' of implant 14', as shown at 26. The posterior surface 14b'partially reflects beam 10' as it passes through posterior surface 14b',which is shown at 28. Finally, beam 10' is reflected as it strikes theposterior surface 12b' of cornea 12', as is shown at 32.

FIG. 5 illustrates the percentage of light reflected as a function ofthe refractive index of the implant produced using the method of thepresent invention. The curve designated at 36 shows the percentage oflight reflected by the cornea and implant together as a function of theindex of refraction of the implant. As can be seen from FIG. 5, if theindex of refraction of the implant equals the index of refraction of thecornea (i.e., 1.376), the percentage of incident light that is reflectedis at the minimum, which is approximately 4%. As it is desirable thatthe total reflection of the cornea and implant together will not differfrom the total reflection of the cornea alone by more than approximately10%, the total reflection of the implant plus cornea should be nogreater than 4.4%. If we find the point on line 36 that gives a totalreflection of 4.4% it can be seen that it corresponds to an index ofrefraction for the implant of approximately 1.52. Since a hydrogel ismostly water and the index of refraction of water is approximately 1.3,the index of refraction of the implant should be at least 1.3.

Therefore it is most desirable for the gel to be used in GIAK surgery tohave an index of refraction greater than 1.3 and less than 1.52.

It is also essential that the absorbance of the injected gel closelymatch the absorbance of the cornea. This will be important if it becomesnecessary to perform later procedures on the eye. If the gel hasdifferent absorbance characteristics, laser ocular surgery andphotocoagulation may not be possible, as the light energy will not havea uniform effect on the gel and the cornea.

Another important characteristic of the injected gel that will affectits performance in the eye is its modulus of elasticity. This subject isdiscussed in an article entitled "Keratoprosthesis: Engineering andSafety Assessment", which was published in the May/June 1993 issue ofRefractive and Corneal Surgery. If the injected implant is stiffer thanthe cornea, it will deform the cornea, while if the cornea is stifferthan the implant, it will deform the implant. For example, akeratoprosthesis which is composed of glass or polymethylmethacrylate(PMMA) is subject to extrusion from cornea, as these relatively hardmaterials have an elastic modulus much greater than that of the cornea.Therefore, to prevent extrusion of the gel from the cornea, its modulusof elasticity must be less than that of the cornea. FIG. 6 shows arepresentation of a cornea for the purpose of locating the site forselecting the proper modulus of elasticity in both the transverse andradial directions. Cornea 40 is composed of a plurality of layers orlamellae 42 which form the stroma 44. The corneal surface is indicatedat 46, while the anterior chamber of the eye is indicated at 48. At theincision site in the cornea for this procedure (approximately 2.5 mmfrom the corneal center), the thickness of the cornea is between 550 and650 microns. At the level at which the annular channel is formed whichis indicated at 50 in FIG. 6, the cornea has both a radial elasticmodulus and a transverse elastic modulus. The radial modulus is directedalong a plane designated by 52 while the transverse modulus is directedalong a plane designated by 54. The transverse modulus is between2.19×10⁴ and 4.12×10⁴ newtons/m², while the radial modulus is between2×10⁶ and 5×10⁶ newtons/m². In order to avoid any problems withextrusion, the gel should have an elastic modulus less than both theradial and the transverse moduli of the cornea.

Other necessary characteristics of an injectable gel for this procedureinclude: the prevention of cell migration into the implant which wouldimpair its removal (if necessary to readjust corneal curvature); and thetransmission of oxygen and other essential nutrients through the gelinto all parts of the eye.

In an experiment using the procedure taught in the aforementioned patentthe sterile crosslinked gel was injected into an annular intrastromalchannel formed between the lamellar layers in the cornea of a rabbit ata distance spaced away from the central corneal region. After thechannel was formed in the cornea, the gel was injected into the channelusing a 19-25 gauge needle. The PEO gel Was shown to be non-toxic to therabbit cornea with an excellent corneal transparency, no surfaceopacification, no extrusion and no migration. Histologically, no giantcells, no necrosis, and a normal keratocyte population near the implantwere found. In addition, the PEO gel was optically transparent in thevisible spectrum and its index of refraction (1.334) was relativelyclose to the corneal refraction index (1.376). The modulus of elasticityof the gel was estimated with a penetrometer at 1.7×10³ newtons/m². Ithas been shown that gel produced by the method of the present inventionremains stable over 22 months in the rabbit cornea. By using a solutionduring preparation of the PEO gel that approximates the electrolyteconcentration or osmotic activity of the cornea, it would be possible tominimize any change in volume of the implant.

Other potential uses are for vitreous substitution and keratophakialenticules. Increasing the PEO concentration increases the gelmechanical strength while decreasing transparency. For example, a 1% PEOsolution irradiated at 5 Mrads will produce a tougher gel which can beused for subcutaneous tissue augmentation procedures performed inplastic and reconstruction surgery, oculoplasty, or other procedureswhere transparency is not necessary. Several experiments have beenconducted in vivo to demonstrate the biocompatibility of this PEO gelwhen injected subcutaneously. Six rabbits received subcutaneousinjection of a PEO gel prepared according to the present invention inthe dorsal area and in the ears. The results showed a good tolerance ofthis material and no apparent degradation of the product after twomonths.

The gamma ray crosslinking process of PEO solutions produces an excessamount of free water (syneresis). The water may be unwanted in certainsurgeries and has to be removed before transferring the gel from thecanister to the syringe. To accomplish this task, the canister wasequipped with a second chamber separated from the first by a fine meshscreen. After the irradiation procedure, the canister was inverted andthe excess water drained into the lower container, while maintaining thecrosslinked PEO in a sterile atmosphere.

In certain instances, it may be difficult to predict at the time ofmanufacture of the PEO what exact shape and size is necessary for aparticular implant. In these situations, the PEO gel can be broken intosmaller pieces (i.e. cracked) with an average particle size ranging fromseveral microns (for use in filling a biological space with greatprecision) to over 1 cm for instances in which large volumes of gel arerequired. The cracking process may be done prior to the implantation orduring the implantation process.

While the invention has been shown and described in terms of a preferredembodiment thereof, it will be understood that this invention is notlimited to this particular embodiment and that many changes andmodifications may be made without departing from the true spirit andscope of the invention as defined in the appended claims.

What is claimed is:
 1. A method for crosslinking poly(ethylene oxide)for use as a gel configured to be implanted within the cornea of the eyeof a mammal for altering the radius of curvature of the cornea,comprising the steps of:dissolving a sample of poly(ethylene oxide)having a molecular weight of approximately 200,000 daltons beforecrosslinking in a saline solution; transferring said poly(ethyleneoxide) to a canister; removing any free oxygen from the canister;replacing the oxygen within the canister with an inert gas; irradiatingsaid canister to crosslink the poly(ethylene oxide) to form a gel havinga modulus of elasticity of less than approximately 4×10⁴ newtons/meter²which is capable of being implanted within the cornea to alter itsradius of curvature.
 2. The method of claim 1, wherein said aqueoussolution comprises a Balanced Salt Solution.
 3. The method of claim 1,wherein said inert gas is Argon.
 4. The method of claim 1, wherein saidcanister is irradiated by a gamma ray source at a dosage of from 2.5 to25 Mrads.
 5. The method of claim 4, wherein said gamma ray source isCobalt
 60. 6. The method of claim 1, wherein the irradiating step tocrosslink said poly(ethylene oxide) also sterilizes said poly(ethyleneoxide).
 7. The method of claim 1, wherein the concentration of thepoly(ethylene oxide) solution formed by dissolving poly(ethylene oxide)in a saline solution is between 0.8% and 8% by weight.
 8. The method ofclaim 1, further comprising the step of draining excess water from thepoly(ethylene oxide) after the irradiating step.
 9. A method ofcrosslinking poly(ethylene oxide) for use as a gel configured to beimplanted within the cornea of the eye of a mammal for altering theradius of curvature of the cornea, comprising the steps of:dissolving asample of poly(ethylene oxide) having a molecular weight ofapproximately 200,000 daltons before crosslinking in a Balanced SaltSolution within a container; filling the container with an inert gas toreplace any oxygen within the container; and irradiating the containerto crosslink the poly(ethylene oxide) to form a gel which can be safelyimplanted within the cornea to alter its radius of curvature.